The invention relates to a position measuring system (path measuring systemxe2x80x94position sensoring system) comprising a transmitter, a sensor which comprises an inductive element to which the transmitter is coupled electromagnetically and an evaluating unit for a sensor signal, wherein the sensor and the transmitter are adapted to be positioned relative to one another.
Position measuring systems of this type are utilised, for example, for measuring the position of pneumatic cylinders, for measuring the position of valves (especially in control loops) or in gripper devices. It is very advantageous in such applications if a relative position between the transmitter and the sensor is measurable absolutely.
In a position sensor known from the state of the art, a secondary coil is wound over a longitudinally extending soft magnetic sensor core. Primary coils are wound around the respective ends of the sensor core. A magnet which is movable along the sensor core is used as a transmitter. A voltage is induced in the secondary coil when current passes through the primary coils. The magnet exerts an influence on this induced voltage thereby causing the magnetic sensor core located in its immediate vicinity to become saturated. The flow of current through the secondary coil is then affected by the relative position between the magnet and the soft magnetic sensor core so that the current through the secondary coil is dependent on the position of the magnet.
In a magneto-inductive sensor-line for magnetically determining the position and/or path of one of the magnets adjacent to the sensor line which is known from DE 43 11 973 A1, a plurality of flat coils that are arranged next to one another and/or above one another is provided on an extensive magnetically conductive layer which can be brought into a state of magnetic saturation, whereby the neighbouring coils form a transmitter-receiver system, and the position of the magnet affects the transmitting/receiving characteristics of the coils thereby allowing its position to be detected.
Inductive position indicators are known from DE 25 11 683 C3 and DE 39 13 861 A1, wherein a ferromagnetic core together with a primary coil traversed by an alternating current form a probe element which itself creates a magnetic flux. This magnetic flux is threaded through a secondary winding and the voltage induced in this winding is dependent on the position of the core.
A displacement sensor is known from FR 2 682 760 A1, wherein a primary circuit and a secondary circuit are arranged on a support. An alternating current is applied to the primary circuit which is coupled to the secondary circuit whereby an alternating voltage is induced therein. This induced voltage is affected by a transmitter consisting of a ferromagnetic material in dependence on the position of the transmitter relative to the secondary circuit.
Based upon the foregoing, the object of the invention is to provide a position measuring system which is of simple construction and can thus be manufactured economically, and which is employable universally.
In accordance with the invention, this object is achieved in the case of a position measuring system as described hereinabove in that the inductive element is coupled to an oscillator which is affected by the Q factor and/or the effective inductance of the inductive element, in that the Q factor and/or the effective inductance of the inductive element is determined by the size of an effective sensor region to which the transmitter is coupled, and/or by the size of an effective transmitter region which is coupled to an effective sensor region, and in that the sensor and/or the transmitter are formed in such a manner that the size of the effective sensor region to which the transmitter is coupled, and/or the size of the effective transmitter region which is coupled to the effective sensor region is dependent on the relative position between the transmitter and the sensor in a direction transverse to a direction of separation therebetween.
Due to the fact that the inductive element is coupled to an oscillator and can affect the parameters of the oscillator such as the amplitude, the phase angle and the frequency thereof by means of its Q factor and/or effective inductance, a location-dependent coupling of the transmitter to the inductive element can be evaluated in a simple manner in that the corresponding parameters of the oscillator are evaluated. The inductive element, which is coupled to the oscillator, is thereby coupled to the oscillator in such a manner that the oscillator itself can be influenced. A special case of coupling between the inductive element and the oscillator arises when the inductive element itself forms the inductance in the oscillator. Thus, in accordance with the invention, it is not a voltage which has been induced in a secondary coil via a primary coil that is measured, but rather, it is the Q factor and/or the effective inductance of the inductive element in the sensor that is evaluated by means of the oscillator. Consequently, energy does not have to be supplied to a primary coil so that the position measuring system in accordance with the invention is constructed in a simpler manner. Moreover, by virtue of the device in accordance with the invention, the transmitter is in the form of a passive element so that, in particular, current does not have to be applied thereto over energy supply lines.
In accordance with the invention, provision is made for the Q factor and/or the effective inductance of the conductive element, which represents a measure for the relative position between the transmitter and the sensor, to be determined from the size of the effective sensor region and/or the size of the effective transmitter region. The sensor signal is thus determined by the geometrical structure of the sensor or the transmitter. The information in regard to the relative position between the transmitter and the sensor and thus the position information or path information regarding the relative position between the transmitter and the sensor is contained in the geometrical form of the effective sensor region or the effective transmitter region which are mutually coupled. In turn, the effective sensor region or the effective transmitter region is determined by the shape of the sensor, and thus especially by that of the inductive element, or by the shape of the transmitter. Consequently, the position measuring system in accordance with the invention is of simple construction and can be produced economically.
The position measuring system in accordance with the invention is employable universally, and may be employed, in particular, in a shaft encoder by virtue of an appropriate shaping of the sensor or the transmitter. Apart from the inductive element, no other secondary coil or the like needs to be provided. Basically, it is sufficient to provide a single inductive element which is constructed in such a manner that an effective sensor region and/or an effective transmitter region, which is coupled to the inductive element, is dependent on the relative position between the transmitter and the sensor. In addition however, it is also possible to provide further inductive elements. For example, difference measurements or sum measurements can be effected in this manner so as to obtain very accurate measurements having a high resolution factor. In accordance with the invention for example, provision may be made for the provision of a plurality of measurement tracks, for example, a measurement track for coarse measurements and a measurement track for fine measurements. Since the location information is in fact contained in the shape of the effective sensor or the effective transmitter region, a plurality of different applications can be implemented by appropriate shaping.
The resolution of the measurement is thereby adapted to be set directly by the shape of the effective sensor region or the effective transmitter region. Resolutions in the order of at least one thousandth of the total path, which the sensor and the transmitter can adopt relative to one another, can thereby be implemented unproblematically.
Since the sensor signal is determined by an effective sensor region and/or by an effective transmitter region so that the sensor signal is determined directly by an effective inductance of the inductive element in the sensor, known evaluating circuits for inductive proximity switches, in which the proximity of a metal object to an oscillator coil is registered by means of a change in amplitude or a change in frequency of the oscillator for example, can be used. Consequently, use may be made of currently available evaluating units. In particular, the position measuring system in accordance with the invention may be provided with a type of evaluating unit independently of the particular shape of the transmitter or the inductive element since, in essence, the evaluating unit is only determining a characteristic value of this inductive element.
It is advantageous if the sensor and/or the transmitter are formed in such a manner that an overlapping region between a projection of an effective transmitter surface area onto the sensor having an effective sensor surface area is dependent on the relative position between the sensor and the transmitter transversely relative to the direction of projection. The shape of the sensor and especially that of the inductive element and/or the shape of the transmitter, by virtue of which the effective sensor surface area or the effective transmitter surface area is in each case determined, then determines the dependency of the coupling between the sensor and the transmitter transversely relative to the direction of projection. In turn, the relative position between the sensor and the transmitter in a direction transverse to the direction of projection (transverse relative to the direction of spacing between the sensor and the transmitter) can be determined from this dependency.
The relative position between the transmitter and the sensor can be determined in a simple manner if the evaluating unit determines a characteristic value of the oscillator. A transmitter, which is made of metal and, in particular, is electrically conductive or magnetic, represents a mutual inductance with regard to the inductive element of the sensor. This coupling of the inductances produces an alteration in the effective inductance of the inductive element. This change in the effective inductances can be measured in a simple manner. In one variant of an embodiment, provision is made for a frequency of the oscillator, to which the inductive element is coupled, to be determined as the characteristic value of the oscillator. The frequency of an LC oscillatory circuit is substantially inversely proportional to the square root of the effective inductance. This can then be determined in a simple manner. This variant is particularly advantageous when the transmitter is a magnet since a relatively large change in inductance can thereby occur, such a change being correspondingly effective on the frequency of the oscillatory circuit especially when a soft magnetic material, which can be brought into a state of saturation, is arranged on the sensor.
In one alternative variant of an embodiment, an amplitude of the oscillator, to which the inductive element is coupled, is determined. The amplitude of an oscillator and especially of an oscillatory circuit is, in turn, dependent on the effective inductance or Q factor of the inductive element in the sensor. It can be determined in a simple manner. In particular, changes in amplitude can be detected which are relatively small. The effective inductance of the inductive element can also be evaluated thereby, especially when the transmitter is a non-magnetic metal.
It is particularly very advantageous if the inductive element is of extensive form (two-dimensional) and especially if it is in the form of a flat coil. The effective sensor region, to which the transmitter is coupled, is then located over an extensive area. Accordingly, the effective sensor region can be deliberately set by the shape of such a flat coil so as to enable the relative position between the sensor and the transmitter to be determined from the size of the effective sensor region. In addition, a flat coil can be produced in a simple manner and it is especially easy to replicate during manufacture; the manufacturing spread is considerably greater in the case of wound three-dimensional coils than it is for flat coils. Hereby, it is especially very advantageous if the inductive element is in the form of a printed coil. The corresponding windings of the coil can thereby be produced on a circuit board in a simple manner, for example, by means of an exposure process. Again, a plurality of coil shapes can thereby be produced so as to obtain a high degree of variability in regard to the application thereof.
It is expedient if the evaluating unit is arranged on a circuit board upon which the inductive element is seated. The evaluating unit and the inductive element are then integrated on a circuit board. The sensor in accordance with the invention can thereby be easily manufactured in an economical manner and, in addition, it is correspondingly easy to install it in a housing for example.
It is expedient if the measurable distance (extent of the path) between the transmitter and the sensor is basically determined by a linear dimension of the inductive element. Due to the shape of the inductive element, it is then possible to appropriately set up a measurement path for a special application, within which path the relative position between the sensor and the transmitter is determinable. A particular setting for the relevant parameters of the position measuring system in accordance with the invention can thereby be achieved from the shape of the inductive element due to the ease with which the inductive element can be produced, especially when it is in the form of a printed coil.
It is especially very advantageous if the transmitter is a passive element and particularly if it is manufactured from an electrically conductive or magnetically conductive material. Here, a passive transmitter is a transmitter which is such that it is not connected to a source of energy but with which an electromagnetic coupling to the inductive element can nevertheless be produced. A constructively simple construction of the position measuring system in accordance with the invention is thereby obtained, this being economical to manufacture and utilise since energy supply lines to the transmitter, which may possibly have to be moved therewith, do not have to be provided. It naturally follows, that a source of energy for the transmitter does not then need to be provided.
In one variant of an embodiment, the transmitter comprises a magnet. The magnetic field of the magnet influences the inductive element and the effect thereof is expressed, in particular, by a change in the effective inductance of the inductive element. In turn, this change in the effective inductance is dependent upon the effective sensor region of the inductive element which is being subjected to the magnetic field. Measurements can even be made through metallic walls with the aid of such a transmitter. For example, the position of a piston provided with such a transmitter can be detected externally through a wall of a pressurised cylinder made of aluminium.
It is expedient hereby if a soft magnetic material is arranged on or in the vicinity of the inductive element. The soft magnetic material may, for example, be a Mu-metal in the form of a foil which has maximum possible magnetic permeability and lowest possible electrical conductivity. The soft magnetic material can be magnetically saturated by the magnetic field of the transmitter and an effective sensor region is then defined by virtue of this local saturation. In turn, the local saturation in the effective sensor region produces a relatively large change in the effective inductance so that this can easily be detected.
In one variant of an embodiment, a soft magnetic material is deposited on a circuit board, for example, on one or both sides thereof, upon which the inductive element is seated. The sensor in accordance with the invention can then be produced in a simple manner. In particular, provision may be made for a circuit board, upon which the inductive element is seated, to have a soft magnetic material wound therearound.
An effective sensor region, which is dependent on the positioning of a transmitter relative to the sensor, can be established if the inductive element is designed such that the shape thereof along a measurement path varies in a direction transverse to said measurement path. Additionally, or as an alternative thereto, it is also possible for the soft magnetic material to be arranged in a form such that the dimensions of its shape transverse to a measurement path vary along said measurement path. Since a state of local saturation can be produced in an effective sensor region by virtue of the soft magnetic material, it is also possible for an effective sensor region to be determined by the shape of the soft magnetic material itself. Thus, in an area outside the soft magnetic material, the effects of the field applied to the sensor will differ from those of a field applied to the soft magnetic material so that the effective sensor region can then be determined by the manner in which the soft magnetic material is deposited. In particular, provision is made for the soft magnetic material to be arranged in the form of a triangle. The transverse dimension of the deposition of soft magnetic material thereby alters along the measurement path and the relative position between the transmitter and the sensor can be determined from this variation in the transverse dimension.
It is particularly very expedient if the inductive element is formed in such a manner that its shape transverse to a measurement path varies along said measurement path. This can be achieved in a simple manner by appropriately shaping the windings of a flat coil. The effective sensor region varies due to the change in the shape thereof transverse to the measurement path. The size of the effective sensor region is, in turn, responsible for the sensor signal and this sensor signal then incorporates the information in regard to the relative position between the sensor and the transmitter. In an easily producible variant of an embodiment, the inductive element comprises triangular windings. A larger surrounding surface is then present in the vicinity of a peak of the triangle than is the case in the vicinity of a base of the triangle. The size of an effective sensor region thereby again varies when a transmitter is coupled to the inductive element.
In one variant of an embodiment, the inductive element extends over an angular range for the purposes of measuring revolutions. If the transmitter is then moved in a circular track about the inductive element, the relative rotational position between the transmitter and the sensor can then be determined. Hereby, the inductive element is constructed in such a manner that an effective sensor region varies over the angular range. It is especially advantageous if the angular range comprises a substantially full circle. The rotational positions can thereby be measured in a complete angular range.
In another embodiment, the transmitter comprises an electrically conductive or a magnetically conductive element. This element is then inductively coupled to the inductive element in the sensor in the form of a mutual inductance and thereby alters the effective inductance of the inductive element. In turn, from this alteration, which is dependent on the relative position between the sensor and the transmitter, this relative position can then of course be determined. It is especially very advantageous hereby, if the projected overlapping area between an effective transmitter region and the inductive element varies along a measurement path. Basically, as has already been explained hereinabove, this variation can be achieved by appropriate construction of the inductive element. In dependence on the application, the transmitter may, for example, be in the form of a tongue, in the form of a hoop which can be moved over the inductive element, in the form of a ring having a rounded or rectangular cross-section for example, or in the form of a tube etc.
In one variant of an embodiment, provision is made for the transmitter to be formed in such a manner that an effective transmitter region, which is coupled to the inductive element, will vary in shape transverse to a measurement path along said measurement path. The effective transmitter region determines the coupling of the transmitter to the sensor. By appropriately shaping this effective transmitter region and, especially by virtue of a variation along the measurement path, the coupling is thereby then made dependent on the relative position between the transmitter and the sensor.
Again, this relative position can then be determined unambiguously from the sensor signal. In one variant of an embodiment, provision is made for the measurement path to be linear. In another variant, the measurement path is circular so that the position measuring system in accordance with the invention is especially adapted to be employed as a shaft encoder. The effective transmitter region is appropriately formed in dependence on the particular variant in which it is employed. In particular, the surface density of the transmitter alters along the measurement path so as to form an effective transmitter region having a variable cross-section.
In one variant of an embodiment, the transmitter is provided with a triangular structure. A variation in an effective transmitter region can thus be achieved in a simple manner by virtue of such a structure. If the triangular structure is arranged in the form of a ring then rotational movements can also be measured therewith.
For example, the effective transmitter region can be formed by means of a coating on the transmitter. The coating material may, for example, be a Mu-metal or a ferrite coating. Provision may also be made for the transmitter to be provided with recesses, especially through-passage openings. Since there is no transmitter material in these recesses, the effective transmitter region is thus dependent on the size and the distribution of the recesses over the transmitter.
In one advantageous variant of an embodiment of a position measuring system in accordance with the invention, the sensor is comprised by an inductive proximity sensor which comprises an oscillatory circuit with an inductive element. Proximity sensors of this type, which are especially in the form of analogue sensors, are known. These can be used with an appropriately constructed transmitter for effecting absolute position measurements for the relative position between the transmitter and the sensor. The relevant shaping of the transmitter i.e. the variation in the effective transmitter region which can be coupled to the proximity sensor, then determines the sensor signal which, in turn, then contains the relevant information in regard to the effective transmitter region and thus in regard to the relative position between the sensor and the transmitter.
It is expedient if a magnetic screening is provided for the inductive element. The inductive elements are thereby protected from stray fields and the like which could be coupled into the inductive element thereby falsifying the measurement signal. The accuracy of measurement is thereby increased due to the provision of such a magnetic screen which may, in particular, be in the form of a magnetic cage.
It is expedient if a plurality of inductive elements are provided. A wide range of possible applications is thereby achieved. For example, difference measurements or sum measurements could be carried out, or, the inductive elements could be arranged in such a manner that the sensor comprises a plurality of measurement tracks which could be used for coarse measurements and fine measurements for example.
In one variant of an embodiment, the sensor comprises a plurality of tracks formed by inductive elements. The tracks could be utilised for difference measurements for example, i.e. a differential system can be formed thereby. Here, the tracks may be formed in opposite senses or in the same sense. In addition, the shape of the tracks may differ so that, for example, one track is constructed for the purpose of making a fine measurement and one track is constructed for making a coarse measurement of the relative position between the sensor and the transmitter.
It is expedient if a plurality of inductive elements are so arranged and interconnected with reference to the transmitter that a position measurement process can be carried out which is substantially independent of the spacing of the transmitter from the sensor. Basically, the electromagnetic coupling of the transmitter to the inductive element is dependent on its spacing from the inductive element. If this spacing alters, then the sensor signal is effected thereby without an alteration in the relative position between the sensor and the transmitter in a direction transverse to that of the spacing. Due to the arrangement of a plurality of inductive elements in accordance with the invention, this spacing-dependency can be compensated so that the relative position between the sensor and the transmitter transverse to the direction of the spacing will be independent of any alteration in the spacing between the sensor and the transmitter along said direction of spacing.
It is expedient hereby if a transmitter is positioned between two mutually spaced inductive elements. In particular here, the inductive elements are directed in opposite senses. However, they could also be directed in the same sense. If a process involving the formation of the difference with regard to the sensor signals from the two inductive elements is carried out, then the spacing-dependency is thereby eliminated. Provision may also be made for a process involving the formation of the sum with regard to the sensor signals from the two inductive elements to be carried out. The spacing between the transmitter and the sensor can of course be determined from the sum signal. Thus, in accordance with the invention, a process for determining the position between the sensor and the transmitter in a direction transverse to the direction of spacing between the transmitter and the sensor can be carried out and, in addition, a positional determination in regard to the spacing between the sensor and the transmitter can be effected i.e. in respect of the height at which the transmitter is disposed above the sensor.
In one advantageous variant of an embodiment, the inductive element is disposed on a flexible support means. In particular thereby, the inductive element is disposed on a flexible foil. The support means together with the inductive element deposited thereon can then be given a certain shape so that, for example, the support means and the inductive element can be adapted to the contours of a track guidance system or the like. It may be advantageous for example, if, due to a wave-like movement of the transmitter, the measurement path is matched to the movement of the transmitter in such a manner that the spacing between the sensor and the transmitter is kept substantially constant. In this case, the inductive element must likewise be arranged such as to have a wave-like shape. This can be achieved with the aid of a flexible support means which is positioned on a corresponding wave-like background. In particular, there is provided a single evaluating unit which carries out the process of forming a difference and/or of forming a sum so as to obtain a corresponding measurement signal; in particular, in accordance with the invention, provision is made for a plurality of sensor elements and more especially a plurality of inductive elements to be associated with an evaluating unit.
It is especially very advantageous if the sensor and/or the transmitter are constructed in such a manner that, by virtue of the corresponding shaping, a certain characteristic curve of the position measuring system will be set up for the sensor signal in dependence on a measurement path. A particular characteristic curve that is desirable for an application can thereby be established.
It is expedient if an error signal is derivable from the evaluating unit, whereby the evaluating unit is adapted to be checked as to whether one or more parameters of the inductive element lie within a tolerance range. In particular, it is checked as to whether the Q factor and/or the effective inductance does not deviate too far above or too far below still tolerable values. Thus, a plausibility check is carried out by means of which, for example, a break in a coil, a short circuit or a failure of the transmitter or the movement thereof away from the measurement region can be detected.